1. Field of the Invention
The present invention is broadly directed to a vibrator system and a method for using a vibrator system to sink pipes or shake equipment. More particularly, the present invention is directed to a mechanical vibrator system which generates frequencies exceeding 150 Hertz for sustained periods of time for earth boring or equipment vibration.
2. Brief Description of the Prior Art
Vibratory devices utilized for generating oscillations in a rigid body and using two identical, symmetrical, contrarotating eccentrics are well known. These contrarotating eccentrics generate a variable oscillatory thrust which alternates in direction 180.degree. along the same axis. This is accomplished because centrifugal forces generated cancel each other when in opposite phase but add to one another when in phase. Such dual-eccentric assemblies are generally achieved by mounting each eccentric mass on an axis and driving it with an independent electric motor. The two axis may be geared together for synchronous rotation, but such gearing may not be necessary if the motors are identical and adequately wired on the same current because the two contrarotating eccentrics will then readily interact and fall into synchronous motion.
The use of these contra-rotating eccentric vibrators for sinking piles and for earth drilling also is a well known technology. These vibrosinkers generally utilize relatively low frequencies, on the order of 1,000 oscillations per minute or 17 Hertz (percussion range) to a maximum of 3,000 to 4,000 oscillations per minute or 50 to 70 Hertz (vibration range). See B. M. Gumenski and N. S. Komarov--(1959) Soil Drilling by Vibration. Translated from Russian, Consultants Bureau, New York, 1961.
Albert G. Bodine in a series of patents issued between 1944 and 1968 advocated the implementation of "sound wave generators" or high-frequency oscillators for earth drilling and pile driving, whereby the vibrator generates oscillations matching the natural frequency of the pipe or of the pile to be driven into the ground. (See for example, U.S. Pat. Nos. 2,350,212; 2,554,005; and 2,682,322.) This concept is often known as "sonic drilling" or "resonant drilling". Bodine's patents are unclear about the exact range of frequencies they endeavour to achieve, except that in U.S. Pat. No. 2,717,763, a frequency of 2,000 Hertz is set forth as an example. Bodine's U.S. Pat. Nos. 2,903,242; 2,975,846; 3,054,463; and 3,194,326 describe various vibrators intended to achieve these very high "sonic" frequencies using sets of contra-rotating eccentrics driven and synchronized by meshed gears.
For many years, particularly in Europe and especially in Russia, the combination of directional vibratory drive (axial reciprocating thrusts) and rotary drive (simultaneous rotation of the drill pipe) has been advocated and implemented for earth drilling. See for example, D. D. Barkan--Methodes de Vibration dans la Construction, translated from Russian by B. Catoire, Dunod, Paris 1963. The reported rates are in the order of 4,000 r.p.m. (67 Hertz) for vibration and 60 to 120 r.p.m. for rotation.
Similarly, in continuation of Bodine's efforts and patents, the firm Hawker-Siddeley, of Canada, has developed a few years ago a "resonant drill" which associate a relatively high frequency, directional vibratory drive (70 to 150 Hertz) with a slow rotary motion (60 revolutions per minute). See: D. R. Dance, 1981--Super drill 150, in Proceedings 3rd Annual Conference of Alaska Placer Mining, University of Alaska, M.I.R.L. Report No. 52, pp. 152-167.
Various researchers have utilized in recent years high-frequency, single-eccentric vibrators of the type known as "concrete vibrators", for driving coring tubes and drilling casings into unconsolidated formations and for extracting casings and piles from the ground. These applications are described in D. E. Lanesky et al, (1979)--A New Approach to Portable Vibracoring Underwater and on Land, Journal of Sedimentary Petrology, vol. 49, pp. 655-657; and also in A. M. Rossfelder et al, (1980), Drilling and Coring Systems for Shallow Water Exploration, Offshore Technology Conference, Houston, pp. 217-221. These "concrete vibrators", used in the building trade for homogenizing and de-aerating poured concrete, consist of a single eccentric directly rotated at high velocity by a built-in electric motor or, more commonly, by a flexible shaft itself rotated by a power unit at some distance. These vibrators generally work at 10,000 r.p.m., i.e. in the range of 170 Hertz. Attached to a earth boring tube, they generate a non-directional standing wave, whereby, simply stated, the tube resonates like an organ pipe and fluidizes the surrounding ground, drastically lowering its skin friction and its resistance to penetration.
It is believed that the meshed-gear eccentric vibrators proposed by Bodine never reached a commercial stage because the high "sonic" frequencies that he was seeking could not be practically achieved with meshed gear transmissions due to inherent mechanical limitations. In fact, the Hawker-Siddeley "resonant drill" only reaches a maximum of 9,000 r.p.m. or 150 Hertz. It is worth mentioning that the recognized range for "sound waves" is from 20 to 20,000 Hertz. This corresponds to the audible frequencies at the maximum intensity sustainable by human hearing, which can otherwise perceive sound waves of 1,500 to 4,000 Hertz when at their faintest intensity. All oscillators discussed so far are therefore within the "sonic" range, but still at its lowest levels.
The drilling units used in the past having a combination of directional vibratory drive and rotary drive have been relying, to the best of our knowledge, on distinct motors and input shafts for impelling the contra-rotating eccentrics on one hand and rotating the pipe on the other. The Hawker-Siddeley Resonant Drill, for example, which combines a directional vibratory drive and a rotary drive, uses hydraulic motors of different characteristics for each.
When a single eccentric mass--either as single eccentric or as set of eccentrics rotating in the same direction--is directly driven through its axle by a flexible shaft, as in the current off-the-shelf "concrete vibrators", the angular velocity of the flexible shaft has to be the same as the one required from the eccentrics. Because of the rapid increase of internal friction and heat losses within the shaft sheathing as velocity increases, such devices are limited in rotational speed and in the distance the eccentric can be placed from the power unit driving the flexible shaft. For example, a 5 HP flexible shaft for a concrete vibrator is limited to a maximum of 10,000 r.p.m. and to a length of about 5 meters.
Finally, it is worth noting that the prior art does not pursue the concept of an eccentric mass rotating at high velocities. This is because the centrifugal force generated is proportional to the square of the circular speed of an eccentric mass and thus a very high and very destructive force can be attained with relatively small mass. To the best of our knowledge, no one has, on a practical scale, been able to consistently and for sustained periods operate, in air, vibrators of the mechanical type at frequencies exceeding about 150 Hertz. It is to be noted that we draw a distinction between a mechanical system described here and a pneumatic vibrator which is also known in the prior art and which has certain limitations even though pneumatic vibrators may reach frequencies exceeding 150 Hertz.